The largest portion of the UN appeal would go to Lebanon ($1.2 billion) and Jordan (nearly $1 billion), where the UN is heading up the response and fears are greatest that the refugees may destabilize their host nations.

Sharp spike

The spike in Syrian departures has shot up: Last year some 50,000 Syrians fled to Jordan. This year some 50,000 a month are leaving -- a mass exodus that has severely taxed Jordan and its government, people, schools, and hospitals.

Already, Jordan hosts nearly half a million Syrian refugees, a figure equal to more than seven percent of the population, officials here say.

"We not only have 1.6 million refugees in the surrounding countries, and several million displaced and in need of assistance inside Syria, but we're not seeing any end to the conflict," says Andrew Harper, head of the UN Refugee Agency in Jordan, describing why the new appeal is so large.

"What we have to do,” Mr. Harper adds, “is not only respond to the immense humanitarian needs of the refugees and others in need from this conflict, but also be prepared for the … continued flight of refugees coming across the borders.”

The UN appeal includes a direct request for funds by Jordan and Lebanon of $830 million for costs imposed on their military forces and public infrastructure.

Jordan's camp the second-largest in world

In Jordan, more than 100,000 Syrians live in the teeming Zaatari refugee camp, now the second-largest in the world. But the vast majority have settled in cities and towns around the country. Syrian refugees may now make up fully half the population of some northern cities.

In Lebanon, some 510,000 refugees already make up more than 10 percent of the population.

Lebanon has elected not to build refugee camps. So the influx is straining the country's decrepit infrastructure and overwhelming its border towns and villages.

Even normally skeptical analysts have begun to worry that the presence of the refugees, combined with the increasing violence along the border, could reignite the sectarian civil war that shook the country from 1975-1990.

Then there is Turkey. Nearly 400,000 refugees fled to government-funded camps early in the Syrian civil war. Reports recently say refugees are being denied entry and are trapped on the border.

UN officials also estimate 4.2 million are displaced in Syria itself and 6.8 million more are in need of humanitarian assistance. The appeal would earmark $1.4 billion for social services in Syria and food aid, as the local farming base has nearly collapsed.

In Jordan, in the dust-choked streets of Zaatari camp, millions have been spent on shelter, food, education and healthcare, yet the refugees there still live a miserable existence. Desert heat, flies and disease, power cuts and water shortages, long lines and minimal medical care are daily problems. Yet many feel they are not being helped at all and are bitter.

"I think if the people in charge want to change the situation, they can do that. It seems like they want us to live in this situation," said a former Syrian nurse-turned-refugee who did not want to give his name. "You can't say that the UN, or all the world, can't solve the problems in the camp."

Numerically, this is the largest refugee crisis in the history of the modern Middle East.

"In terms of numbers, I think this is the most significant flow since the end of World War 2 and the Palestinian crisis," says Dawn Chatty, head of refugee studies at Oxford University in Great Britain.

In 1948, the Palestinians who fled the creation of the State of Israel numbered between 750,000 and a million. Over the years, Palestinians have become the largest displaced population in the world.

At the height of the Iraq war in 2008, the UN had actually registered fewer than 300,000 refugees, spread between Jordan, Syria, Egypt, and the Gulf countries. The numbers have decreased since then as many were resettled in Western nations and others returned to Iraq.

In comparative global and historical terms, Syria's crisis rates high. The crisis may have already spawned more refugees than the exodus of Cambodians in the 1970s, documented in the popular film The Killing Fields, or in the subsequent flight of refugees from communist Vietnam.

In the decade after the Soviet invasion of Afghanistan, refugee numbers spiked from 1.7 million in Iran and Pakistan to 6.3 million by 1990.

“I have been involved with humanitarian operations for 30 years, and it’s difficult to recall a refugee crisis that was larger in scale, that grew so quickly or which took place is such a politically complex regional environment,” writes Jeff Crisp, Head of UNHCR’s Evaluation Service, in an e-mail to the Monitor from northern Iraq, where he was conducting an assessment of the response to the crisis.

“The international community has devoted unprecedented levels of support to the Syrian refugee emergency. But unfortunately it is just not enough. The crisis is getting worse on a daily basis."

Today’s UN appeal follows a request at the end of 2012 for $1.5 billion aimed at the first half of 2013. The funds were slow to arrive; currently about 75 percent of the total has been delivered.

UN agencies worry that if the international community does not provide sufficient funding, the nations hosting refugees will start to close their borders.

In May, the number of refugees arriving in Jordan dropped precipitously, for reasons the UN still does not fully understand. The floods of refugees early in 2013 have fallen to a trickle of a few hundred or less in recent weeks.

The Jordanian government says the slowdown is because of intense fighting in southern Syria, which has prevented refugees from crossing the border. However some accounts from refugees and people in Syria have claimed that the Jordanian military have begun turning refugees away.

A video that went viral here in the last week of May appears to show Jordanian military forces blocking the path of a long line of refugees trying to cross the border. Although 30,000 Syrians still entered the country over the course of the month, the slowdown has raised fears that the host country's patience will wear thin without a greater commitment from the international community.

In Jordan, the scale of the unmet needs is growing by the day.

"The amount of funds needed to keep such a large number of people protected and assisted should not be underestimated," says Harper. "[In Jordan] we're having to build another refugee camp the same size as Zaatari. We're going to have to provide accommodation, tents; we're going to have to provide water and food; we're going to have to build health facilities and schools. You can just imagine having to establish a city for 500,000 people or a million people--which will be the case by the end of the year--in any country in the world: the cost is just enormous."

For refugees outside of camps, the UN provides assistance only to the most vulnerable. Even so, officials in Jordan say they are worried the numbers are reaching a breaking point.

"We should be providing cash assistance to some 30,000 families, the majority of them headed by women or elderly, who've actually got no means to provide funds for themselves," Harper says. "We're only providing funding for 11,000, and we've only got money for that [until] the end of June. So what happens to those people [after] June?"